Apparatus for centrifugal casting

ABSTRACT

The process of centrifugally casting an article such as a tire or the like from a curable or hardenable liquid polymeric material, which process includes the steps of selecting a mold and placing a core within the mold which core is hollow and/or is readily deformable under pressure but which has sufficient memory to resume its original position when the pressure is removed. The article being formed is formed between the core and the mold. The space between the core and the mold is filled with the curable liquid material of which the article is to be formed and the hollow core is also filled with a liquid material. The liquid material in the hollow core, the material from which the core is constructed and the curable liquid material all have about the same specific gravity. The mold and core are rotated to centrifugally cast the article which is formed of the curable liquid material. The deformable core permits easy removal from the completed article and the matching of the specific gravities as aforementioned keeps the deformable core from distorting during the centrifugal casting operation. The order of introducing the liquid material into the core, introducing the curable liquid into the space between the mold and core and rotation of the mold and core can be varied within the limits as set forth in the following description. Variations in the structure for accomplishing the principle of matching specific gravities are illustrated and described.

United States Patent [1 1 McGillvary i 1 APPARATUS FOR CENTRIFUGALCASTING Daniel Ross McGillvary. Jackson Township. Stark County. Ohio[73] Assignee: The Firestone Tire & Rubber Company, Akron, Ohio [22]Filed: Jan. 26, 1973 {2|} Appl. No: 327,041

Related US. Application Data [62] Division of Scr. No. 159,893. July 6,I97], Pat. No.

[75] Inventor:

Primary ExuminerRobert L. Spicer, Jr.

{57] ABSTRACT The process of centrifugally casting an article such as 5]Apr. 22, 1975 a tire or the like from a curable or hardenable liquidpolymeric material. which process includes the steps of selecting a moldand placing a core within the mold which core is hollow and/or isreadily deformable under pressure but which has sufficient memory toresume its original position when the pressure is removed. The articlebeing formed is formed between the core and the mold. The space betweenthe core and the mold is filled with the curable liquid material ofwhich the article is to be formed and the hollow core is also filledwith a liquid material. The liquid material in the hollow core, thematerial from which the core is constructed and the curable liquidmaterial all have about the same specific gravity. The mold and core arerotated to centrifugally cast the article which is formed of the curableliquid material. The deformable core permits easy removal from thecompleted article and the matching of the specific gravities asaforementioned keeps the deformable core from distorting during thecentrifugal casting operation. The order of introducing the liquidmaterial into the core. introducing the curable liquid into the spacebetween the mold and core and rotation of the mold and core can bevaried within the limits as set forth in the following description.Variations in the structure for ac complishing the principle of matchingspecific gravities are illustrated and described.

11 Claims, 8 Drawing Figures VACUUM PATENTEnAPazzms sum 2 or 3 FIG. 2

1 APPARATUS FOR GENTRIFUGAL CASTING This is a division of US Pat.application Ser. No. 159,893 filed July 6, l97l. now US. Pat. No.3,751.55 l.

Centrifugal casting has been employed for some time and is a usefultechnique commonly used by many diverse industries. lt is frequentlyutilized by the rubber and plastics industry to avoid air entrapment inthe fabrication of various shapes and centrifugal casting is a techniquenow used to some degree in the casting of vehicle tires.

The technique commonly employed involves the use of a metal moldspinning symmetrically on an axis of rotation. ln the case of vehicletires and other items which are generally hollow. a core must beemployed which is usually made of metal or other rigid material. Thecore is usually made up of many pieces which are heavy, difficult toinsert and remove, and cause uneven cure of the molded item as well ashaving numerous other disadvantages. There are many occasions when it isdesirable to use a flexible, elastic, and possibly inflatable core toreplace the rigid core, however, the problem with these type ofstructures is to keep the flexible core in position and have it retainits original shape under the centrifugal force developed by the rotationof the mold.

The primary object of the present invention is to provide, in thecentrifugal casting of tires and similar items, a process which willenable the use of a flexible and resiliently yieldable core which can beeasily removed from the finally cured and finished tire and which willnot be distorted by the centrifugal force which is encountered in thecentrifugal casting process. For the sake of brevity the underlyingprinciple will hereinafter be sometimes referred to as the matching ofspecific gravities," however, the entire understanding of the principleswill best be understood by reference to the drawings which show the bestmode contemplated for carrying out the teachings of the invention.

Before proceeding to the drawings and a more precise description ofcarrying out the present invention, a brief discussion of some of theprinciples involved in the present invention are in order. If one, forthe sake of discussion, takes the example of a mold which is filled witha liquid and spun or rotated, it can be said that the liquid will have ahigher pressure within itself the further one proceeds from the centerof rotation. The liquid will also exert that same pressure gradient onall parts with which it is in contact and in the specific case ofcasting a tire this will be both the core and the mold. The pressure,and therefore the distorting force, on a flexible core, at any givenpoint is a function of the depth of the liquid, rotational speed,specific gravity, and the position of rotation. The present inventioncontemplates the designing of a flexible core of rubber or some similarmaterial that is sufficiently rigid to maintain its molded shape when itis standing alone and at rest (free standing) but which normally wouldbe distorted under the centrifugal force encountered in centrifugalcasting. Next let it be assumed that the specific gravity of thematerial of the core is designed to match the specific gravity of thecurable liquid which is to be cast and also for the purpose of thisdiscussion let it be assumed that a liquid is available which will beused to fill the hollow core (balancing liquid) which is of the samespecific gravity as the core material and the castable curable material.

The mold is filled with the curable liquid polymeric material in theprecise amount needed to produce the finished article, in the spacebetween the mold and the core. The hollow core is then filled with thebalancing liquid, so that the same level of curable liquid material ismaintained as in the balancing liquid. Now as the mold and core are spunor rotated, there will be no ef fective distorting force on the coresince all forces are now automatically in balance because the sameforces are being exerted on one side of the core as on the other. Thecore, since it is constructed of a material which has sufficient memory"to return to its original shape, will be free to assume its originalmolded shape. It will thus be seen that if we have the same force on theoutside of the core as on the inside, then there will be no effectivedistorting force in the centrifugal casting process.

Other objects and a fuller understanding of this invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is an elevational view partially in section of the centrifugalcasting apparatus incorporating the teachings of the present inventionand utilized for carrying out the process of the present invention;

FIG. 2 is a view similar to FIG. 1 but showing the parts of theapparatus shown in FIG. 1 moved to a dif' ferent position so as to beable to discharge a completed vehicle tire;

FIG. 3 is a fragmentary view in section showing a modification of theapparatus illustrated in FIGS. 1 and 2 with the modification beingprimarily in the construction of the core;

FIGS. 3A and 3B illustrate how the lower surface of the diaphragm may bevaried;

FIG. 4 is a fragmentary view in section showing a still furthermodification of the apparatus illustrated in FIGS. 1 and 2 and showing avariation in the core construction;

FIG. 5 is a fragmentary view in section illustrating how the teachingsof the present invention can be applied to the construction of a mold asdistinguished from a core as illustrated in FIGS. 1 through 4; and

FIG. 6 is a fragmentary view in section showing a combination utilizingthe present teachings in the mold as well as the core or in other words,a combination of FIGS. 1 and 5.

Referring specifically to FIG. 1 of the drawings, there is shown theapparatus of the present invention which is capable of conveniently andeconomically carrying out the process of the present invention. Shown inFIG. 1 are first and second mounting pedestals 22 and 23 whichrespectively support rotatable shafts 24 and 25. Either one or both ofthe pedestals may incorporate means for rotatably driving the shafts 24and 25, however, this means has not been specifically shown because itforms no particular part of the present invention. A first centrifugalmold half 29 is secured to the shaft 24 in the manner shown and in asimilar manner a second centrifugal mold half 30 is rotatably secured toshaft 25. Each of the mold halves 29 and 30 are provided with flangesboth of which are identified by the reference numeral 31 and the flangesare adapted to be held together by bolts 32. This means of connectingthe mold halves together has been shown for convenience,

however, it will be appreciated by those skilled in the art that manydifferent means may be utilized to hold the halves together which may bemore quickly disconnected than the bolts which have been shown.

An air cylinder 35 is mounted on pedestal 23 and includes a piston rod36 which is connected to a collar 37 which in turn is secured forlongitudinal movement to shaft 25. This means of connection permitsuninhibited rotational movement of the shaft 25 with respect to the rod36 and air cylinder 35, however, the air cylinder when actuated enablesthe mold half to be moved relative to the mold half 29 as shown by thedifferent positions of the same in FIGS. 1 and 2.

Mounted within the confines of the mold halves 29 and 30 is a flexiblecore member 48, in this particular instance constructed ofa rubbermaterial, and the construction of the core 48 is such that when noforces or pressures are applied thereto it has sufficient resiliency ormemory to assume the position as shown in FIG. 1. The completed tireshape is designed so as to have its outer shape defined by the innerwalls of the mold halves 29 and 30 and its inner shape defined by theouter surface or outer wall of the flexible core 48. The hardenableliquid polymeric material has been indicated by the reference numeral 50in FIG. I and as shown in FIG. I this polymeric material has hardened toits finely cured state, but those skilled in the art will appreciatethat before hardening this material is in liquid form. An opening 52 isprovided in the hub of mold half 29 and is for the purpose of permittingthe introduction of the hardenable liquid polymeric material prior tocuring to its solid state.

The apparatus illustrated in FIG. 1 also includes conduit means 46 whichis provided in shaft 24 and pedestal 22 which conduit means leads to avacuum source 43 or a pump 40 depending upon the position ofa valve 44.The pump 40 is provided in the conduit means which leads to astabilizing liquid material source 39 and is for the purpose of pumpinga precise amount of stabilizing liquid material 54 when desired, to thehol low flexible core and which will be discussed more fullyhereinafter. It will be noted particularly in FIG. 2 as well as beingshown in FIG. I, that the shaft 24 is provided with a depressed orfemale portion 57 which is adapted to receive a male portion 58 which isprovided on the end of shaft 25. The reason for this is that after thevehicle tire has been completed in the apparatus of the presentinvention and in accordance with the teachings of the process, the moldhalves must be separated and the shafts must separate so that thecompleted tire can be removed. FIG. 1 as illustrated shows the flexiblecore 48 filled with the precise amount of stabilizing liquid material 54desired in accordance with the teachings of the present invention.

The hardenable liquid polymeric material 50 which is introduced throughopening 52 to produce the completed vehicle tire will, for the sake ofexample in the present teachings, be a polyurethane material, which mayhave a specific gravity in the range of from 1.03 to L30. Theconstruction of the flexible core 48 from the rubber material will thenalso be designed and produced so that its specific gravity is, from apractical technical standpoint, the same as the specific gravity of thematerial 50. In turn the stabilizing liquid material 54 which can beessentially any liquid, is designed so that its specific gravity is thesame as the specific gravity of the material of the core 48 and of thehardenable liquid polymeric material 50. The stabilizing liquid material54 might be a water and salt mixture, an oil or even a polyurethanematerial but with the polymerizing end groups inactivated so that itremains in a liquid form. With respect to the core 48 it may beconstructed with or without supporting cords, may be of a perfectlyplain gum stock or for example a 60/40 synthetic to natural rubberblend. The core should in effect be free standing and should hold itsshape for example within 0.02 inch.

The production of a vehicle tire in the apparatus of FIG. I and inaccordance with the teachings of the process of the present inventionwill best be understood from a brief description of the steps involved.With the apparatus essentially in the position shown in FIG. I with theexception that no hardenable liquid polymeric material 50 is yet presentand the flexible core 48 is in the inactive or withdrawn condition asshown in FIG. 2, the centrifugal mold comprising the mold halves 29 and30 is rotatively driven and the precise amount of hardenable liquidpolymeric material 50 is introduced into the mold through opening 52.The valve 44 is shifted to the position of FIG. I and a precise amountof stabilizing liquid material 54 is pumped into the hollow flexiblecore 48 through conduit means 46 so that the level of stabilizing liquidmaterial 54 in the rotating condition of the mold is at the same heightor the same level as the amount or the level of the hardenable liquidpolymeric material. This level has been indicated by the dot-dash line60 in FIG. 1 and if desired, the appa ratus may have a transparent peephole 61 through which the level can be observed by observing the core48. It will be appreciated that the vacuum is released and thestabilizing liquid material causes the flexible core to distend itselfor move to the position shown in FIG. 1. It will thus be seen that thecentrifugal forces in the stabilizing liquid material 54, in thehardenable liquid polymeric material 50 and in the flexible core 48, areessentially the same at any given distance and there is in effect no netforce tending to distort the flexible core. The core has sufficientmemory or resiliency to assume its molded or formed condition and itwill reassume this condition in which it is shown in FIG. I and as aresult the centrifugal molding process can continue until the vehicletire has been sufficiently cured so that the molding process can bestopped and the tire removed from the mold. It will also be appreciatedby those skilled in the art that the stabilizing liquid material 54which is utilized within the core can also be utilized in curing thetire as it is being centrifugally cast. To this end a heating means 62has been illustrated in the conduit which leads to the liquid source 39so that any desired heat can be imparted to the stabilizing liquidmaterial. After the tire has been formed, the action of the pump 40 isreversed causing the stabilising liquid material 54 to be withdrawn fromthe flexible core and pumped back into the source 39. A flexible plasticor rubber tube 47 is connected to conduit means 46 and is for thepurpose of assuring withdrawal of all the liquid material 54. The valve44 is then manipulated to the position shown in FIG. 2, which causes theflexible core to be withdrawn or retracted somewhat to the position ofFIG. 2 and then air cylinder 35 is retracted, after bolts 32 have beenremoved. which causes mold half 30 to be moved to the position shown inFIG. 2 at which time the completed tire can be conveniently removed byslightly further distorting the flexible core 48 with no damage to thesame. After this has been accomplished the parts are returned to theposition of FIG. I and the process is ready to be repeated.

FIG. 3 is a modification of the teachings of the present invention and amodification occurring principally in the construction of the previouslyidentified flexible core 48. This modification enables one to eliminatethe necessity of the stabilizing liquid 54. The mold halves have beenidentified by the same reference numerals as used in FIGS. 1 and 2 aswell as the other structure where it is the same. The core which isutilized in FIG. 3 includes first and second shaped sides 65 and 66constructed of a metal material which has sufficient rigidity tocounteract any of the forces encountered in the centrifugal castingprocess of the present invention and utilizing the hardenable liquidpolymeric material which has been discussed hereinabove. A rubberdiaphragm 68 is attached to the opposite lower ends of the sides 65 and66 as shown in FIG. 3. Conduit 46 connects the interior of the core ofFIG. 3 to an air pressure source 7] through valve 44. It will thus beseen that the centrifugal casting process is carried out in essentiallythe same manner as discussed above in conjunction with FIGS. 1 and 2,however, in this particular circumstance, the distorting forces producedby the centrifugal force in the hardenable liquid polymeric material 50are balanced above the rubber diaphragm 68 by the introduction of airpressure from the air pressure source 71. To this air pressure will beadded the force resulting from the rotating diaphragm 68. It will beappreciated that a rather delicate control must be maintained on the airpressure supply introduced into the hollow core because the distortingforce within the liquid 50 is a function of the rotational speed of themold and as a result when the rotational speed of the mold changes, thedistorting force within the liquid will change. The sides 65 and 66 maybe made in arcuate segments if desired, to facilitate removal of thefinished tire.

FIGS. 3A and 3B illustrate how the shape of the diaphragm 68 shown inFIG. 3 can be varied. These diaphragms have been identified by thereference numerals 69 and 70 and like 68 are of the same specificgravity as the liquid material 50. Their upper surface (the surfaceexposed to the air pressure) must be flat as in the case of diaphragm68, however, their lower sur' faces may be shaped as desired to give thedesired contour to the inside of the tire. The lower surfaces ofdiaphragms 69 and 70 are identified by reference numerals 72 and 73respectively.

FIG. 4 shows a still further modification of the invention asillustrated in FIGS. I through 3 and in this embodiment there isillustrated a solid rubber" core. In this particular regard the solidrubber core has been illustrated by the reference numeral 74 and it isshown mounted on a hub 75 which hub extends in a generallycircumferentially extending direction and which can be readilydisassembled so that the core can be more conveniently collapsed to airin the removal of the finally finished vehicle tire. In this regard, thehub 75 can be made in circumferentially extending sections. The onlyrequirement in this particular construction is that the specific gravityof the material of the solid rubber core 74 be the same as the specificgravity of the hardenable liquid polymeric material 50 which isintroduced thereinto during the centrifugal casting process. Whether oneconsiders the embodiments of FIGS. I, 3 or 4, it

can always be said that one is balancing the specific gravity of thecore so as to conform to the specific gravity of the curable liquidmaterial.

FIG. 5 illustrates the teachings of the present invention as applied tothe mold rather than to the core. Many prior art disclosures havealluded to or envisioned the use of cheap rubber molds within a lightmetal covering so as to provide support, however, the rubber form ormold which has previously been envisioned must either be stiff enough toprevent growth, or be reinforced or supported in some way. If they arestiff enough or have a high degree of reinforcement they are in and ofthemselves difficult and expensive to manufacture. FIG. 5 illustrateshow the present teachings of matching specific gravities can be appliedto a mold. The rubber of the mold need only be strong enough to maintainits shape while spinning and not tear while disassembling the part whichhas been formed therein. In this regard FIG. 5 shows a mold 77 formed inthe shape as shown, of a relatively inexpensive rubber material which issupported in a rather inexact manner by a thin metal housing 78. Thematerial of construction of the mold 77 is designed so as to be of thesame specific gravity of the hardenable liquid polymeric material whichis to be cast thereinto during the centrifugal casting process. It willtherefore be seen that the corners or any surface portions on theinternal surface of the mold 77 will not have a tendency to distortbecause of the centrifugal force applied thereto and exerted by thepolymeric material because the specific gravities are the same and atthe same positions from the radius of rotation and will be exerting thesame counteracting forces. The tire 79 produced is a solid rubber tireas shown.

The modification shown in FIG. 6 is a combination of the teachings ofFIGS. I and 5. Stated another way, the mold of FIG. 5 has beenincorporated into the apparatus of FIG. I. To this extent the moldhalves 77 are contained in housing 78 and flexible core 48 is locatedtherein, and is adapted to be filled with liquid material 54. Theapparatus function in the same manner as described in conjunction withFIGS. I and 2. It will be appreciated that the mold of FIG. 6 may alsobe used in combination with the cores of FIGS. 3 and 4.

The resinous materials which are capable of being utilized in accordancewith the teachings of the present invention are practically unlimited,the principal consideration being that the resin, whatever its nature,be a liquid prior to curing. The resin must be curable and may be curedby heat, by addition of a cross-linking agent, or by catalyst, bycondensation, by chemical reaction, or any combination thereof. Tiresconstructed in accordance with the teachings of this invention arepreferably constructed with what are commonly referred to aspolyurethane resins. For the most part these are of the polyether type;however, polyestertype urethanes may also be used for this purpose. Thecomposition of these materials are well known and reference may be hadfor example to US. Pat. Nos. 2,852,483 and 2,814,605 and 2,814,606 forspecific examples of polyether-polyurethanes and polyesterpolyurethanes.These materials are readily available commercially as prepolymers havinga viscosity comparable to that of honey. The polyurethanes generallyhave specific gravities in the range of from 1.03 to 1.30. The resins ofthe present invention would normally be considered to be an organiccompound of high molecular weight above say about 1,000, containingrepeating units achieved by a process encompassing any mechanismresulting in an increase in molecular weight, and including for example,polymerization, condensation, addition graft polymerization, blockpolymerization, cross-linking, solvation etc., or any combinationthereof. The resin or resins hereof are polymeric or res inous materialsin which the extent of polymerization is insufficient to convert thematerial from a normally liquid state to a jelled or fully cured solidstate. Such curable liquid resins include therefore, prepolymers,plastisols, A and B stage resins, latently catalyzed resins, etc Theresins are in their final cured state, that is their solid state, whenany of the foregoing processes of increase in molecular weight occur andsuch increase is terminated either by exhaustion of reactant materi als,utilization of the entire amount of cross-linking agent, removal ofheat, use of chain stopping reactants, or the like. As indicated above,the final cured products are preferably elastomeric, that is, in theirfinal cured state they possess resilience or elasticity.

Although this invention has been described in its preferred form andpreferred practice with a certain degree of particularity, it isunderstood that the present disclosure of the preferred form andpreferred practice has been made only by way of example and thatnumerous changes in the details of construction and the combination andarrangement of parts and steps may be restored to without departing fromthe spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, a hollow flexible coremounted within said mold which core is deformable under pressure but hassufficient memory to resume its original position when the pressure isremoved, a curable liquid material in the space between the core andmold which material is to form the finished article, a stabilizingliquid material in said hollow core, said material of construction ofsaid core, said curable liquid material and said stabilizing liquidmaterial all having substantially the same specific gravity.

2. Apparatus as claimed in claim 1, wherein the article is a vehicletire and the mold is constructed of a relatively rigid material.

3. Apparatus as claimed in claim 2, wherein the rela- 8 tively rigidmaterial is metal.

4. Apparatus as claimed in claim 1, wherein the article is a vehicletire and the mold is mounted for rotation by a substantiallyhorizontally disposed shaft, means are provided for introducing andremoving the stabilizing liquid material from the hollow core.

5. Apparatus as claimed in claim 4, wherein means are provided forheating the stabilizing liquid material prior to being introduced intothe hollow core.

6. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, a solid and flexiblecore mounted within said mold which core is deformable under pressurebut has sufficient memory to resume its original position when thepressure is removed, a curable liquid material in the space between thecore and mold which material is to form the finished article, saidmaterial of construction of said core and said curable liquid materialhaving sub stantially the same specific gravity.

7. Apparatus as claimed in claim 6, wherein the article is a vehicletire and the mold is constructed of a relatively rigid material.

8. Apparatus as claimed in claim 6, wherein the relatively rigidmaterial is metal.

9. Apparatus as claimed in claim 6, wherein the article is a vehicletire and the mold is mounted for rotation by a substantiallyhorizontally disposed shaft.

10. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, said mold being solidand is deformable under pressure but has sufficient memory to resume itsoriginal position when the pressure is removed, a curable liquidmaterial in the mold which material is to form the finished article,said material of construction of said mold and said curable liquidmaterial having substantially the same specific gravity.

11. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, a flexible diaphragmmounted a spaced distance from the mold to define a space, means forintroducing a curable liquid material of which the article is to beformed between the mold and the diaphragm in the space, and means forintroducing a gas under pressure to the side of the diaphragm oppositethe side adapted to receive the curable liquid material.

1. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, a hollow flexible coremounted within said mold which core is deformable under pressure but hassufficient memory to resume its original position when the pressure isremoved, a curable liquid material in the space between the core andmold which material is to form the finished article, a stabilizingliquid material in said hollow core, said material of construction ofsaid core, said curable liquid material and said stabilizing liquidmaterial all having substantially the same specific gravity. 1.Apparatus for centrifugally casting an article including in combinationa mold, means for rotating said mold, a hollow flexible core mountedwithin said mold which core is deformable under pressure but hassufficient memory to resume its original position when the pressure isremoved, a curable liquid material in the space between the core andmold which material is to form the finished article, a stabilizingliquid material in said hollow core, said material of construction ofsaid core, said curable liquid material and said stabilizing liquidmaterial all having substantially the same specific gravity. 2.Apparatus as claimed in claim 1, wherein the article is a vehicle tireand the mold is constructed of a relatively rigid material.
 3. Apparatusas claimed in claim 2, wherein the relatively rigid material is metal.4. Apparatus as claimed in claim 1, wherein the article is a vehicletire and the mold is mounted for rotation by a substantiallyhorizontally disposed shaft, means are provided for introducing andremoving the stabilizing liquid material from the hollow core. 5.Apparatus as claimed in claim 4, wherein means are provided for heatingthe stabilizing liquid material prior to being introduced into thehollow core.
 6. Apparatus for centrifugally casting an article includingin combination a mold, means for rotating said mold, a solid andflexible core mounted within said mold which core is deformable underpressure but has sufficient memory to resume its original position whenthe pressure is removed, a curable liquid material in the space betweenthe core and mold which material is to form the finished article, saidmaterial of construction of said core and said curable liquid materialhaving substantially the same specific gravity.
 7. Apparatus as claimedin claim 6, wherein the article is a vehicle tire and the mold isconstructed of a relatively rigid material.
 8. Apparatus as claimed inclaim 6, wherein the relatively rigid material is metal.
 9. Apparatus asclaimed in claim 6, wherein the article is a vehicle tire and the moldis mounted for rotation by a substantially horizontally disposed shaft.10. Apparatus for centrifugally casting an article including incombination a mold, means for rotating said mold, said mold being solidand is deformable under pressure but has sufficient memory to resume itsoriginal position when the pressure is removed, a curable liquidmaterial in the mold which material is to form the finished article,said material of construction of said mold and said curable liquidmaterial having substantially the same specific gravity.